Switch MHTML generation to the new callbacks.

BUG=714018

Change-Id: I33cd6842055c959af25e286b1962c6f902b35971
Reviewed-on: https://chromium-review.googlesource.com/974284
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545087}
11 files changed